EP0097759A1 - Barrier anodizing of aluminium and aluminium alloy substrates - Google Patents

Barrier anodizing of aluminium and aluminium alloy substrates Download PDF

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Publication number
EP0097759A1
EP0097759A1 EP83100927A EP83100927A EP0097759A1 EP 0097759 A1 EP0097759 A1 EP 0097759A1 EP 83100927 A EP83100927 A EP 83100927A EP 83100927 A EP83100927 A EP 83100927A EP 0097759 A1 EP0097759 A1 EP 0097759A1
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EP
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Prior art keywords
barrier
aluminium
anodizing
current
current density
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EP83100927A
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German (de)
French (fr)
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EP0097759B1 (en
Inventor
George Achilles Condas
Saad Kamel Doss
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International Business Machines Corp
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International Business Machines Corp
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/04—Anodisation of aluminium or alloys based thereon

Definitions

  • This invention relates to processes for producing barrier anodized layers on aluminium and aluminium alloy substrates.
  • barrier anodic coatings for capacitors.
  • Barrier anodization generally refers to anodic coatings that are essentially pore-free and are generally of the order of about 10 metres in thickness, whereas conventional anodic coatings are about 10 -5 metres in thickness.
  • Most of the prior art in barrier anodizing has dealt with high purity aluminium, and not much is known about barrier anodizing of either aluminium alloys or large areas of pure aluminium with pore-free anodized films.
  • barrier anodizing electrolytes for aluminium such as (a) aqueous boric acid-borax solutions, (b) aqueous or semi-aqueous solutions containing citrate of tartrate ions, and (c) solutions of ammonium pentaborate decahydrate in ethylene glycol may be suitable for high purity aluminium, and are generally used for barrier anodizing of aluminium.
  • Aluminium alloys referred to herein are designated by the four digit designation system established by the Aluminium Association and generally known in the art.
  • US-A-3,864,219 discloses a barrier anodizing process for aluminium and aluminium alloys in which the anodizing current is maintained at a level between 0.1 and 10 ma/cm 2 .
  • US-A-3,846,261 discloses a barrier anodizing process using alternating electrical current, but no mention is made of the current densities employed.
  • barrier anodizing of an aluminium or aluminium alloy substrate is performed using barrier anodizing current densities which are at least much higher than those employed in the prior art, these high current densities being employed for a shorter length of time than the lower current densities of the prior art.
  • barrier anodized layers produced by the present technique have exceptional sealing and adhesion characteristics. These are particularly important, for example, in the treatment of aluminium alloy substrates for use in magnetic recording disks where it is critical that the metal substrate be sealed to prevent corrosion and that this sealing layer have good adhesion to the underlying substrate.
  • Barrier anodized substrates made by the process according to the present invention result in good adhesion between the barrier layer and an overlying magnetic layer, such as epoxy/phenolic/magnetic pigment mixtures.
  • Barrier anodizing is carried out in a slightly acidic to neutral (5 ⁇ pH ⁇ 7) bath having a DC power supply connected between a cathode and an anode on which the barrier layer is to be formed.
  • the anode is a disk substrate composed of an aluminium alloy, such as the type 5086 alloy of aluminium and magnesium
  • the barrier layer is a layer of alumina formed on the substrate surface.
  • the phenomenon of barrier anodizing can be represented graphically by a curve plotting anodizing current versus time, with the initial current remaining at a relatively steady level until a time t 1 , called the barrier formation time, at which time the current begins to decrease as a result of the increased resistance of the essentially non-conductive barrier layer.
  • the current is a function of the current density and the substrate surface area.
  • a barrier of thickness d l is formed that is related to the voltage V 1 of the applied power by the equation where K is the growth constant common to aluminium of approximately 1.4nm/v.
  • the quality of the barrier film formed is improved with shorter barrier formation times t 1 .
  • the anodizing current density J 1 is maintained much higher than in the prior art and at at least 20ma/cm 2 , so that the barrier formation time t 1 is much shorter than in the prior art, resulting in greatly improved barrier films.
  • the current density is preferably maintained in the range from 20 to 3000ma/cm 2 .
  • One example of a method according to the present invention is as follows.
  • a bath was prepared using 3% by weight of tartaric acid in deionized water.
  • the pH of the bath was adjusted to approximately 7 by the addition of ammonium hydroxide.
  • This solution was in a tank having a stainless steel cathode, with a 345.6mm (14 inch) aluminium disk substrate with a central hole forming part of the anode.
  • An adjustable DC power supply applied 300 volts between the cathode and anode at a current of 30 amperes, resulting in a current density of 20ma/cm 2 . Both faces of the disk were anodised. Using a small part cut from a disk, a current density of 300 ma/cm 2 was obtained. This current density was maintained constant until barrier formation time t i , which occurred after 4 seconds. In contrast, with an anodizing current density of 3ma/cm 2 , this barrier formation time was 400 seconds.
  • Hardness tests conducted on the anodized layers produced in accordance with the present invention showed a surface having a Knopp hardness of 480 kG/cm 2 with a 5 gramme load, which is harder than sealed layers produced on some current 5086 disk substrates by other methods.
  • Ten 5086 substrates were barrier anodized at each of the following voltages: 50, 100, 150, 200 and 250V, two disks per voltage setting.
  • the corresponding alumina thicknesses were 70, 140, 210, 280 and 350nm, respectively.
  • the current density for the anodizing at 150 volts was 20ma/cm 2 . All parts were then coated with a magnetic coating, cure baked, buffed to about a 0.10314 ⁇ m (41 microinch) surface finish,, and washed.
  • the adhesion test for some current magnetic disks requires severe buffing, until the substrate inner diameter (ID) is .exposed. The remaining magnetic layer (paint) is then microscopically (X50 - 200) examined for tears. Acceptable adhesion requires no visible tears.
  • a disk for each barrier forming voltage (five disks) described above was buffed for adhesion testing. The disks whose barriers were processed at 50 and 100 volts had a few small tears. Disks. processed at 150 and higher voltages had much better (and acceptable) adhesion. They had no tears whatsoever.
  • An obvious conclusion was that adhesion increases with forming voltage, and acceptable adhesion occurs at barrier voltages of 150 and greater. Very high voltages (greater than 250 V) increase barrier surface roughness; hence, forming voltage in the range of 150 to 200 are recommended, although voltages up to 500 volts may be employed.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacturing Of Magnetic Record Carriers (AREA)
  • Electrochemical Coating By Surface Reaction (AREA)
  • Magnetic Record Carriers (AREA)

Abstract

In a barrier anodizing process for an aluminium or aluminium alloy substrate, the anodizing current density is maintained between 20 and 300 ma/cm2 and the current is terminated after a period of time not appreciably exceeding the barrier formation time. The voltage is preferably maintained between 150 and 200 volts.

Description

  • This invention relates to processes for producing barrier anodized layers on aluminium and aluminium alloy substrates.
  • The use of barrier anodic coatings for capacitors is well known. Barrier anodization generally refers to anodic coatings that are essentially pore-free and are generally of the order of about 10 metres in thickness, whereas conventional anodic coatings are about 10-5 metres in thickness. Most of the prior art in barrier anodizing has dealt with high purity aluminium, and not much is known about barrier anodizing of either aluminium alloys or large areas of pure aluminium with pore-free anodized films.
  • Known barrier anodizing electrolytes for aluminium such as (a) aqueous boric acid-borax solutions, (b) aqueous or semi-aqueous solutions containing citrate of tartrate ions, and (c) solutions of ammonium pentaborate decahydrate in ethylene glycol may be suitable for high purity aluminium, and are generally used for barrier anodizing of aluminium. Aluminium alloys referred to herein are designated by the four digit designation system established by the Aluminium Association and generally known in the art.
  • US-A-3,864,219 discloses a barrier anodizing process for aluminium and aluminium alloys in which the anodizing current is maintained at a level between 0.1 and 10 ma/cm2.
  • US-A-3,846,261 discloses a barrier anodizing process using alternating electrical current, but no mention is made of the current densities employed.
  • In accordance with the present invention, barrier anodizing of an aluminium or aluminium alloy substrate is performed using barrier anodizing current densities which are at least much higher than those employed in the prior art, these high current densities being employed for a shorter length of time than the lower current densities of the prior art. It has been found that barrier anodized layers produced by the present technique have exceptional sealing and adhesion characteristics. These are particularly important, for example, in the treatment of aluminium alloy substrates for use in magnetic recording disks where it is critical that the metal substrate be sealed to prevent corrosion and that this sealing layer have good adhesion to the underlying substrate. Barrier anodized substrates made by the process according to the present invention result in good adhesion between the barrier layer and an overlying magnetic layer, such as epoxy/phenolic/magnetic pigment mixtures.
  • The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described by way of example.
  • Barrier anodizing is carried out in a slightly acidic to neutral (5<pH<7) bath having a DC power supply connected between a cathode and an anode on which the barrier layer is to be formed. In a preferred form, the anode is a disk substrate composed of an aluminium alloy, such as the type 5086 alloy of aluminium and magnesium, and the barrier layer is a layer of alumina formed on the substrate surface. The phenomenon of barrier anodizing can be represented graphically by a curve plotting anodizing current versus time, with the initial current remaining at a relatively steady level until a time t1, called the barrier formation time, at which time the current begins to decrease as a result of the increased resistance of the essentially non-conductive barrier layer. The current is a function of the current density and the substrate surface area. At barrier formation time tl, a barrier of thickness dl is formed that is related to the voltage V1 of the applied power by the equation
    Figure imgb0001
    where K is the growth constant common to aluminium of approximately 1.4nm/v.
  • It has now been discovered, by use of an ionic drift model, that
    Figure imgb0002
    where B is a constant related to K and J1 is the initial current density.
  • It has now also been observed that the quality of the barrier film formed is improved with shorter barrier formation times t1. In accordance with the present invention, the anodizing current density J1 is maintained much higher than in the prior art and at at least 20ma/cm2, so that the barrier formation time t1 is much shorter than in the prior art, resulting in greatly improved barrier films. The current density is preferably maintained in the range from 20 to 3000ma/cm2.
  • One example of a method according to the present invention is as follows.
  • A bath was prepared using 3% by weight of tartaric acid in deionized water. The pH of the bath was adjusted to approximately 7 by the addition of ammonium hydroxide. This solution was in a tank having a stainless steel cathode, with a 345.6mm (14 inch) aluminium disk substrate with a central hole forming part of the anode. An adjustable DC power supply applied 300 volts between the cathode and anode at a current of 30 amperes, resulting in a current density of 20ma/cm2. Both faces of the disk were anodised. Using a small part cut from a disk, a current density of 300 ma/cm2 was obtained. This current density was maintained constant until barrier formation time ti, which occurred after 4 seconds. In contrast, with an anodizing current density of 3ma/cm2, this barrier formation time was 400 seconds.
  • Further, examination of the barrier anodized surface visually and by means of a scanning electron microscope (SEM) revealed a virtually defect-free surface. In contrast to this, comparative samples produced at a barrier anodizing current density of 3ma/cm2 and at the same voltage revealed a significant number of defects and voids in the anodized surface.
  • Hardness tests conducted on the anodized layers produced in accordance with the present invention showed a surface having a Knopp hardness of 480 kG/cm2 with a 5 gramme load, which is harder than sealed layers produced on some current 5086 disk substrates by other methods.
  • To test the suitability of the disk substrates produced by the present process as a base for the application of a liquid magnetic coating, the following adhesion tests were conducted.
  • Ten 5086 substrates were barrier anodized at each of the following voltages: 50, 100, 150, 200 and 250V, two disks per voltage setting. The corresponding alumina thicknesses were 70, 140, 210, 280 and 350nm, respectively. The current density for the anodizing at 150 volts was 20ma/cm2. All parts were then coated with a magnetic coating, cure baked, buffed to about a 0.10314µm (41 microinch) surface finish,, and washed.
  • The adhesion test for some current magnetic disks requires severe buffing, until the substrate inner diameter (ID) is .exposed. The remaining magnetic layer (paint) is then microscopically (X50 - 200) examined for tears. Acceptable adhesion requires no visible tears. A disk for each barrier forming voltage (five disks) described above was buffed for adhesion testing. The disks whose barriers were processed at 50 and 100 volts had a few small tears. Disks. processed at 150 and higher voltages had much better (and acceptable) adhesion. They had no tears whatsoever. An obvious conclusion was that adhesion increases with forming voltage, and acceptable adhesion occurs at barrier voltages of 150 and greater. Very high voltages (greater than 250 V) increase barrier surface roughness; hence, forming voltage in the range of 150 to 200 are recommended, although voltages up to 500 volts may be employed.

Claims (5)

1 A method of providing a barrier layer on the surface of an aluminium or aluminium alloy substrate by barrier anodizing, characterised in that the barrier anodizing is performed at an anodizing current density of at least 20ma/cm2 for a period of time not appreciably exceeding the barrier formation time.
2 A method according to claim 1, in which the barrier anodizing current density is maintained between 20 and 300ma/cm2.
3 A method according to claim 1 or 2, in which the voltage in the barrier anodizing is maintained between 150 and 500 volts.
4 A method according to claim 3, in which the voltage is maintained between 150 and 200 volts.
5 A method according to any preceding claim, in which the barrier anodizing is conducted in a bath of tartaric acid having a pH of between 5 and 7.
EP83100927A 1982-06-28 1983-02-01 Barrier anodizing of aluminium and aluminium alloy substrates Expired EP0097759B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US06/392,840 US4400246A (en) 1982-06-28 1982-06-28 Process for applying barrier layer anodic coatings
US392840 1982-06-28

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EP0097759A1 true EP0097759A1 (en) 1984-01-11
EP0097759B1 EP0097759B1 (en) 1987-06-03

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DE (1) DE3371918D1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0212668A1 (en) * 1985-08-29 1987-03-04 Chemal GmbH &amp; Co., KG Process for the uniform electrolytic colouring of anodized aluminium or aluminium alloys

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4981672A (en) * 1983-06-27 1991-01-01 Voltaix, Inc. Composite coating for electrochemical electrode and method
JPS60164927A (en) * 1984-02-07 1985-08-28 Nippon Light Metal Co Ltd Production of alumite substrate for high-density magnetic recording material

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CH565871A5 (en) * 1971-05-18 1975-08-29 Isovolta
CH584767A5 (en) * 1971-12-17 1977-02-15 Henkel & Cie Gmbh
US4188270A (en) * 1978-09-08 1980-02-12 Akiyoshi Kataoka Process for electrolytically forming glossy film on articles of aluminum or alloy thereof
US4211619A (en) * 1978-03-16 1980-07-08 Hoechst Aktiengesellschaft Process for anodically oxidizing aluminum and use of the material so prepared as a printing plate support

Family Cites Families (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2408910A (en) * 1942-07-27 1946-10-08 Sprague Electric Co Electrical condenser
JPS4923737A (en) * 1972-06-29 1974-03-02
JPS5335514B2 (en) * 1973-05-29 1978-09-27
JPS54143738A (en) * 1978-04-30 1979-11-09 Setsuo Tomita Highhspeed anodizing of aluminum
JPS5513918A (en) * 1978-07-15 1980-01-31 Matsushita Electric Works Ltd Radiator
JPS5521503A (en) * 1978-07-28 1980-02-15 Canon Inc Coloring of aluminium
JPS5789497A (en) * 1980-09-26 1982-06-03 Hoechst Co American Anodic oxidation of plate like, sheet like or strip like material made of aluminum or aluminum alloy
JPS57210996A (en) * 1981-06-20 1982-12-24 Yamaha Motor Co Ltd High speed anodization method

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CH565871A5 (en) * 1971-05-18 1975-08-29 Isovolta
CH584767A5 (en) * 1971-12-17 1977-02-15 Henkel & Cie Gmbh
US4211619A (en) * 1978-03-16 1980-07-08 Hoechst Aktiengesellschaft Process for anodically oxidizing aluminum and use of the material so prepared as a printing plate support
US4188270A (en) * 1978-09-08 1980-02-12 Akiyoshi Kataoka Process for electrolytically forming glossy film on articles of aluminum or alloy thereof

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0212668A1 (en) * 1985-08-29 1987-03-04 Chemal GmbH &amp; Co., KG Process for the uniform electrolytic colouring of anodized aluminium or aluminium alloys

Also Published As

Publication number Publication date
DE3371918D1 (en) 1987-07-09
JPS6230276B2 (en) 1987-07-01
US4400246A (en) 1983-08-23
JPS599194A (en) 1984-01-18
EP0097759B1 (en) 1987-06-03

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